This invention relates to a magnetic servo system and, more particularly, to a magnetic disk having a data surface for positioning a magnetic head on the magnetic disk using servo information embedded in the data surface of the magnetic disk.
As a magnetic disk data surface servo system, there has been known a sector servo system and an index servo system. In the sector servo system, a plurality of servo sectors are formed in the data surface of a magnetic disk. Each servo sector is formed such that servo information is embedded therein, and the magnetic head is positioned according to servo information signals obtained from each servo section through the magnetic head. In this servo system, the servo information signals are obtained over the entire circumference of the disk, so that it is not influenced by the eccentricity of the disk. For this reason, this servo system is well suited for a magnetic disk device adapted for replaceable disks. Further, since the servo information is obtained over the entire circumference of the disk, accurate positioning of the head can be obtained even if there is drop-out of some servo information. However, since in this servo system servo information is embedded, basically the magnetic disk is regarded as having a hard sector structure. Therefore, if it is intended to format the disk at the magnetic disk device employing the servo system, a gap has to be provided between adjacent sectors to absorb rotational variations caused during rotation of the disk. The provision of such gap greatly reduces the formatting efficiency.
The index servo system is advantageous for increasing the track density of the disk of a small-size rigid magnetic disk device which does not require a capability for tracking eccentricity of a magnetic disk. In this servo system, an index area is provided on part of the data surface of the magnetic disk where servo information is embedded. The head position is corrected once per rotation of disk using the servo information in this area. Thus, the formatting of soft sectors is made possible as in a non-servo magnetic disk device. In this servo system, however, accurate positioning of the head becomes impossible if any drop-out of servo information occurs due to such causes as external noise or defects of a disk, since only the servo information in the index area can be utilized.
An improved index servo system is disclosed in a copending U.S. patent application Ser. No. 841,820 entitled "Magnetic Disk Having Data Area and Index Servo Area and Servo System for Positioning Read/Write Head on Magnetic Disk", filed Mar. 30, 1986 and assigned to the same assignee as this application.